Together with an increase in the volume of transmission traffic in recent years, there are growing demands for next-generation optical transmission systems with a transmission rate over 100 Giga bit per second (Gbps). In these next-generation optical transmission systems, it is also desired to achieve a transmission distance longer than in conventional systems.
However, in terms of achieving a transmission rate over 100 Gbps, conventional optical dispersion compensation technologies have reached their limit for compensation performance. For this reason, a digital coherent optical receiver using a digital coherent receiver system, which is capable of compensating for waveform distortion in a wide range with high accuracy, is desired. The digital coherent receiver system is a system in which information on both optical amplitude and phase is converted to electric signals for reception.
Japanese Laid-open Patent Publication No. 2008-271182 and Japanese National Publication of International Patent Application No. 2012-520614 disclose examples of the related art.
By using a local oscillator having the same frequency as a received signal (laser light), a digital coherent optical receiver mixes the received signal and local oscillation light using an optical phase hybrid, and extracts electric fields (amplitudes) and phase components of the received signal. The digital coherent optical receiver additionally performs photoelectric conversion, analog-to-digital (A/D) conversion, and digital signal processing to recover transmission data from the received signal.
However, the frequency of a received signal does not completely match the frequency of a local oscillator, for example, because of individual differences among local oscillators. In order to compensate for the mismatch between the frequencies, the digital coherent optical receiver includes a frequency offset compensation unit that detects and corrects a frequency offset, which is a frequency difference. Unfortunately, noise that increases with the transmission distance makes it difficult for the frequency offset compensation unit to accurately detect a frequency offset. As a result, a phase shift or the like occurs, making it difficult to correctly recover transmission data.
A digital signal processing unit included in the digital coherent optical receiver equalizes waveform distortion by using a distortion equalization unit, and compensates for a shift (offset) in frequency between the light source of the transmitter and the light source of the receiver by using the frequency offset compensation unit. The digital signal processing unit further detects and corrects a phase error (carrier phase) of a code signal point by using the carrier phase recovery unit. Then, the digital signal processing unit performs identification determination and error correction by using an identification determination unit to recover transmission data.
The phase error of the code signal point in the carrier phase recovery unit does not vary in time when compensation for a frequency offset is ideally performed. However, it is known that, if noise increases, the carrier phase recovery unit makes a false detection of a phase error, because of a residual frequency offset, so that a phase slip occurs, and the carrier phase recovery unit fails to correct an error, thereby giving rise to a transmission error. The residual frequency offset is a residue that remains after the frequency offset has been compensated for by the frequency offset compensation unit.
Conventionally, in order to decrease the amount of noise so as to avoid a situation in which there is a residual frequency offset, it has been desired, for example, to decrease the transmission distance. This inhibits freedom of system design. To address this, a digital signal processing unit of a digital coherent optical receiver that is capable of reducing a residual frequency offset and compensating for waveform distortion with high accuracy is desired.